1. Field of the Invention
The present invention relates to a glass article for use as an optical waveguide fiber and more particularly to an optical waveguide fiber, the core of which is doped with a chalcogenide element (i.e., sulfur, selenium) to increase the refractive index of the core.
2. Prior Art
Optical waveguides are low loss filaments which have been recently developed as the transmission medium for high capacity optical communication systems. To efficiently achieve this result, it is required that optical waveguides possess non-zero numerical aperture (NA) values as defined by the equation: NA=(RI2corexe2x88x92RI2clad)1/2, where RIcore is the refractive index of the core and RIclad is the refractive index of the clad at the wavelength of interest. In order to obtain such values, the equation requires that the refractive index (RI) of the core of the filament be greater than the RI of the cladding layer of the filament.
Furthermore, optical waveguides are often grouped into cables or bundles to provide capacity; each individual fiber typically being tied into its own light source. It is also possible to bundle the fibers to provide redundancy in case of fiber breakage and to transmit a greater amount of the light generated by a source.
Silica is the most important oxide material in telecommunications and optical applications, in part because it can be obtained in exceptionally high purity via chemical vapor deposition methods. Prior art methods have in common the combustion of precursor halides or organometallic compounds in a fuel-air or fuel-oxygen flame. The fine soot that results is laid down on a substrate and consolidated into a bulk glass. Other components may be added with silicon, provided that their vapor pressures are high enough to be delivered to the burner. Well known examples include phosphorous, boron, germanium, and titanium. In each case the metal is combined with oxygen, obtained in the flame from the oxidizer and from the ambient atmosphere. Limited amounts of fluorine can be added as well, usually by flowing fluorine over the soot during the consolidation. However, addition of anions other than fluorine is extremely difficult.
Metal oxides are added to silica to obtain new optical properties. Titanium and germanium are added, for example, to increase the refractive index and/or to obtain a photorefractive effect for writing gratings. Aluminum is added to germanosilicate-based 1.5 micrometer erbium doped amplifier glasses to obtain broader, flatter gain spectra.
As is well known to practitioners in the technology of optical waveguide fibers for telecommunications, the refractive index of the core material must be higher than that of the cladding material in order to support transmission of an optical signal. As noted supra, this is typically performed by doping with such materials as titanium and germanium. These and other materials are known to raise the refractive index values of silica glass, but these materials have been detrimental to other properties, extremely difficult to incorporate, or expensive for applications such as telecommunication transmissions, especially with regard to unwanted absorption or intrinsic scattering of signals. Due to the severe limitations of useful metallic dopants and methods of incorporating anionic species, advancement in the art of telecommunication via waveguide fiber optics has been hampered.
High quality optical waveguides must meet many stringent requirements before they can have commercial viability. Some of these physical requirements may include minimal loss of propagated light signal, high physical strength, and low coefficient of expansion. The process of manufacturing optical waveguides place additional constraints on the types of materials that can be utilized. Such constraints may include tight viscosity range, minimal volatility of dopants, maintenance circularity during rod preform and redrawing operations, similar coefficients of expansion of the core and the cladding, core softening point temperature below or near to the cladding softening point temperature, and high tensile strength during redrawing.
Additionally, depending on the desire to have single mode or multi-mode waveguides, the numerical aperture (NA) must have specific values for a particular core radius. The NA value is derived from an equation that relates refractive indices of the core and the cladding materials, so constraints on NA values cause constraints on materials that can be selected for the core or the cladding compositions.
The stringent optical requirements placed on the transmission medium to be employed in optical communication systems has negated the use of glasses obtained from melted precursors, since attenuation therein due to both scattering and impurity absorption tends to be too high. Thus, unique methods had to be developed for preparing very high purity glasses in filamentary form.
Ideally, the consolidated optical fiber preform should have uniform characteristics along its length. In practice, however, it has been found that the consolidation process results in xe2x80x9caxial trendsxe2x80x9d along the length of the consolidated preform such that fiber produced from the tip of the preform has different properties from properties of fiber produced from the middle of the preform. Similarly, fiber produced from the middle has different properties from properties of fiber produced from the handle end. Metallic dopants such as germanium exacerbate this problem. Therefore, alternative dopants would be desirable.
As can be seen from the number of design and process limitations, compositions for an optical waveguide are severely restricted. There is a strong desire to identify and utilize novel compositions and novel manufacturing processes.
One aspect relates to an optical waveguide containing at least 0.01 mol, more preferably at least 0.05 mol percent and most preferably at least 0.1 mol percent of a chalcogenide such as sulfur. The waveguide preferably also consists primarily of silica, for example in an amount greater than 85 mol percent. In a preferred embodiment, the optical waveguide is an optical fiber comprising a core and a cladding region, the refractive index of said core and clad being configured with respect to one another so that the core is capable of guiding light.
Prefered chalcogenide elements to be incorporated into the silicate glass include sulfur and selenium, the most preferred being sulfur. In the most preferred embodiment, the waveguide or fiber contains at least 0.05, and more preferably more than 0.1 mol percent sulfur.
The fiber or other waveguide may also contain an element selected from the group consisting of phosphorous, aluminum, and boron and mixtures thereof, either to alter the refractive index of the material or the optical characteristics of the material. Likewise, the waveguide may additionally contain metal ions selected from the group consisting of germanium, titanium, zirconium, lanthanum, arsenic, and antimony and mixtures thereof.
The optical waveguide may additionally include lanthanide metals to produce optical activity (e.g. for amplifier or laser glass).
Another aspect of the present invention relates to a method of making an optical waveguide preform containing sulfur therein. In the method, a plasma is ignited inside a substrate tube (for example, as in plasma enhanced chemical vapor deposition) in the presence of a chalcogenide containing precursor compound and a silica forming precursor compound, under conditions which are effective to deposit and thereby depositing a glassy deposit comprised of silica doped with said chalcogenide material on the inside of said tube.
During said depositing step, the tube is preferably heated to a temperature of greater than about 1100xc2x0 C. The depositing step preferably also takes place in the presence of an amount of oxygen which is approximately equal to or less than the amount of oxygen needed to convert the amount of silicon atoms present to a stoichiometric silica glassy deposit. In this way, an amount other aspect of the present invention relates to novel doped silica core compositions wherein a portion of the oxygen in the silica is replaced with either sulfur, selenium or tellurium using plasma enhanced chemical vapor deposition (PECVD). These compositions have higher refractive indices than silica, low coefficients of expansion, high optical transparency, and appropriate viscosity and softening points to make them ideal candidates for use as optical waveguides.
Deposition of one material onto a second material has been one approach to creating a core-clad glass optical waveguide. Traditionally, this is performed using conventional vapor deposition techniques such as Inside Vapor Deposition (IVD) and Outside Vapor Deposition (IVD). However, these conventional techniques utilized a flame hydrolysis process that typically caused unwanted decomposition of reactant materials leading to byproducts or physical defects in the nascent glass article. We have discovered that plasma enhanced chemical vapor deposition (PECVD) eliminates these problems and allows incorporation of previously unavailable materials, in particular chalcogen elements.
Plasma laydown differs from conventional IVD or OVD approaches in that reactants (metal ions and anions) are combined together from separate sources in a plasma, rather than in a flame. In addition, the materials go directly from the plasma to fully dense glass, thereby controlling off-gas of erstwhile volatile components during the consolidation (firing) step. This capability allows the possibility of replacing the oxygen with other anions in order to create either new compositions or oxygen deficient pure silica.
The present invention results in a number of advantages over prior art methods. For example, the compositions and methods disclosed herein provide an alternative to optical fibers composed of germanium-doped silica core glass for use in telecommunication technology.
Using the methods disclosed herein, a silicate core glass can be formed which has higher refractive index than silica, wherein the increased refractive index is obtained via a nonmetallic dopant, such as sulfur, selenium or tellurium. The most preferred of such dopants is sulfur.
Such methods and the resultant compositions are expected to be useful in making optical fiber for optical gratings and optical amplifiers.
A photosensitive fiber light guide, whose core, according to the invention, is doped with sulfur, enjoys the following additional advantages in comparison with the prior art. The low concentration of dopant ensures low optical losses in the infrared range. Waveguide properties of the fiber light guide made according to the invention are close to the properties of a standard light guide, thereby facilitating their connection. Moreover, the claimed light guide, as shown by experiments, possesses high photosensitivity at a wavelength of 193 nm without additional hydrogen treatment, which complicates the process of recording photoinduced structures and causes a deterioration in their temperature stability. A fairly large induced change in refractive index (xcex94n greater than 1xc3x9710xe2x88x923) is achieved at relatively low dose (D less than 1 kJ/cm2) and energy density (1-100 mJ/cm2) of ultraviolet radiation. The process of change in refractive index is initiated by one-photon absorption at the indicated wavelength, for which reason the requirements on spatial uniformity of the beam of exciting laser radiation during recording of the photoinduced structure is not as high as in the case of a process based on two-photon absorption.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.